Error with std::getline

Question

So this is the code:

#include <iostream>
#include <fstream>
#include <vector>
#include <string>

using namespace std;

void print_file(const ifstream& dat_in)
{
    if(!dat_in.is_open())
        throw ios_base::failure("file not open");

    string buffer;
    while(getline(dat_in, buffer));  //error here
}

int main()
{
    ifstream dat_in("name_of_the_file.txt");

    try{
        print_file(dat_in);
    }
    catch(ios_base::failure exc){
        cout << exc.what() << endl;
    }
}

And I get an error that no instance of overloaded function std::getline matches the argument list. I did this line of code a thousand of times, what is the problem now ...

3 IntelliSense: no instance of overloaded function "getline" matches the argument list
        argument types are: (const std::ifstream, std::string)  
Error 1   error C2665: 'std::getline' : none of the 2 overloads could convert all the argument types

Answer 1

the problem is here

void print_file(const ifstream& dat_in)

getline necessarily changes the stream that is passed-in. So change the above to (remove const)

void print_file(ifstream& dat_in)

Answer 2

The culprit is the const:

 void print_file(const std::ifstream& dat_in)
              // ^^^^^

Of course the std::ifstream's state is changed when reading data from it, thus it cannot be const in that context. You should simply change your function signature to

 void print_file(std::ifstream& dat_in)

to get this working.

---

BTW the function name print_file is pretty confusing for a function that actually reads from a file.

Answer 3

Your code is passing a reference to a const ifstream parameter as the first parameter to std::getline(). Since std::getline() modifies its input stream parameter, it cannot have a const reference as the first parameter.

The error message from the compiler included a list of all the parameters, and it should've indicated that the first parameter is a const reference.

Answer 4

根据经验，传递并返回所有流类型作为引用，既不是const也不是by-value。请记住，const指的是对象，而不是文件，即使文件是只读文件，对象也有许多可能改变的东西。